This invention relates generally to the technical field of electrical transmission and interconnection and more particularly to interconnecting an electronic circuit composed of bare monolithic integrated electronic circuit chips and other passive and active devices and elements.
Monolithic integrated electronic circuit chips as generally known are a bare chip inside a package with the input-output ports or pads electrically connected to contacts that lead outside the package enclosure and require further coupling to external circuits with the use of a printed circuit board for their proper operation. Such coupling is necessary for supplying power for such circuits to operate, for applying input signals to such circuits, and for transmitting output signals from such circuits to other devices. The small, rectangularly shaped, conductive areas, frequently referred to in the trade as pads of the monolithic electronic circuit chips are generally arranged about the periphery of one of the chip's planar surfaces, are used for coupling the chip to conductive leads of the enclosing package which in turn couples via soldered leads to external circuits of the printed circuit board which are frequently referred to as traces.
Presently, the various types of mechanical enclosures in which such monolithic integrated electric circuit chips are packaged include a plurality of conductor leads adapted to interconnect such pads of the chip by individually connected wires to corresponding electrical conductors of the package which in turn connect with solder or pins to sites on the traces of a printed circuit board.
The method employed most widely to establish the interconnection between the monolithic integrated electronic circuits chip's pads and the conductors of the package enclosure includes bonding a plurality of individual electrical wires between pads and other conductors included in the package enclosing the chip. Most commonly, each such individual wire is attached to the monolithic electronic circuit chip's pads manually either by thermo-compression bonding or by ultrasonic bonding. The use of Very High Speed Integrated Circuits, (VHSIC) which drastically increases the density and quantity of semiconductor elements possible on a chip and increases the number of functional monolithic electronic circuit chips that can be manufactured on a single semiconductor wafer, brings about a real need to reduce the size of the input/output pads and to reduce the size of connectors to these pads to accommodate the 100 to 400 leads required to access the higher density circuits. The reduction in connector size made possible by this invention will allow reduction in pad size to allow full advantage of VHSIC processing with resultant economic benefits.
Further, in the assembly of advanced monolithic integrated electronic circuit chips using the semiconducting compound, Gallium Arsenide, merely heating these devices during an assembly process has a detrimental effect on the electrical performance of such chips.
A process is needed that connects in a simple fashion, all necessary electronic and electrical elements of a system with minimum labor and without the application of heat. The present invention covers this need.
Recognizing that significant economies might be achieved if it were possible to simultaneously form all the interconnections of on electronic system made up of numerous monolithic integrated electronic circuit chip's (hereinafter called electronic circuit chip) pads simultaneously in a single operation, attempts have been made to develop methods and machines to perform such interconnections employing a plurality of conductors; however, manual bonding employing a plurality of individual wires presently remains the most widely used method.
In addition to requiring relatively expensive, manual methods for interconnecting the pads of the electronic circuit chip with conductors included in chip package, generally that package's shape and the structure such as pins used to interconnect that package to the printed circuit board's traces occupy a significant amount of space on the board. The board space has an area cost equal to that of the electronic chip and packaging.
Further, the most widely used electronic circuit chip package also exhibits significant differences among the electrical characteristics of the various conductors from the chip to the printed circuit which can upset the timing of signals critical to the proper operation of an electronic circuit chip.
Responding to these various deficiencies in the present integrated circuit packaging technology, a new assembly for and method of precisely interconnecting numerous unpackaged electronic circuit chips to form an electronic system was invented. This new assembly and method, which is described in Ser. No. 854,920, included an elastomeric pressure member containing an aperture to receive a chip and a second elastomeric pressure member to go below a chip placed in the aperture. Conductive leads with conductors on each end and supported on a photo-resist membrane inside a frame of aluminum placed between the chip and lower elastomeric pressure member connect the pads of the chip and the traces of a printed circuit board when a metallic cover sized to compress the elastomeric pressure member about 30% was urged toward the circuit board to complete the assembly.
Briefly, in one preferred embodiment of the present invention, one elastomeric pressure member is placed above one or more chips which in turn are placed so that the chip pads contact the connector on one of the electrical leads supported on photo-resist with the other end of these electrical leads having a connector to connect to traces on a substrate when the one elastomeric pressure member is urged toward the substrate by a metallic cover.
A particular advantage of this new electronic circuit chip package is that it efficiently dissipates heat generated by the chips operation to the surrounding atmosphere if the assembly's cover is fabricated from a high thermal conductivity material such as a metal.
While such efficient heat dissipation is necessary to the operation of certain types of integrated circuits such as those employing emitter-coupled logic circuits, high performance linear circuits, and high performance random access memory, not all types of monolithic integrated circuit chips require such efficient heat dissipation. For example, integrated circuit chips employing complimentary MOS circuits ("CMOS") generally do not generate a great deal of heat nor do they frequently require an electrical connection to their substrate; however, more advanced integrated circuit chips made with Gallium Arsenide do require good heat dissipation and have improved life if a cold pressure connection of the chip pads to external leads is made.
Another advantage of the present invention is that it provides a simpler and less costly assembly for and method of connecting electronic circuit chips to a substrate or printed circuit board to allow the formation of distributed systems operating in close proximity with physical sensors and actuators. This is desireable for more effective and reliable control and operation of machines such as hypersonic aircraft, missiles, and submarines.
Another advantage of the present invention is that it requires only a small area of a substrate or printed circuit board for coupling electronic circuit chips thereto.
Still another advantage of the present invention is that it provides conductors having closely matched characteristics for coupling an electronic circuit chip to a substrate or printed circuit board. The substrate or printed circuit board can also be formed to couple the electronic circuit chip with such devices as terminating resistors, grounded shielded coaxial conductors that are tuned for matched impedances at each end of the transmission line to transmit two-way signals between chips and graded in-route to achieve a higher performance, low noise, low reflection signal between interfaces.
An object of the present invention is to provide a simpler and less costly assembly for and method of connecting a number of unpackaged electronic circuit chips to a substrate or printed circuit board so that they may function in a more complete circuit.
Another object of the present invention is to provide an assembly for connecting a number of unpackaged electronic circuit chips to a printed circuit board in such a manner that the total assembly occupies a small area on the substrate or printed circuit board.
Another object of the present invention is to provide an assembly for connecting a number of electronic circuit chips to a substrate or printed circuit board wherein closely matched characteristics among the numerous conductors allows a comparable electron travel time during operation.
Another object of the present invention is to provide an assembly for connecting a number of electronic circuit chips to a substrate or printed circuit board which is suitable for use with chips requiring a large number of individual connections.
Another object of the present invention is to provide an assembly for connecting a number of electronic circuit chips to a substrate or printed circuit board containing sensors, actuators, and effectors and other integrated elements that become a part of the board by process and are in close proximity and in communication with the chips assembly.
Another object of the present invention is to provide conducting leads with connectors on either end supported on photo-resist in an aluminum tape along with an elastomeric pressure member such that any electronic circuit chips regardless of metalization system can be pressure connected by the methods of this invention.
Another object of the present invention is to provide an interconnection means for high density chips having pad sizes as small as eight microns on the side and spaced as close as ten microns between adjacent pad centers thereby allowing use of closely spaced pads on chips to give an improvement in productivity for electronic circuit manufacturers.
Still another object of the present invention is to provide a manufacturing method that will allow total automation of an otherwise labor intensive portion of industry. Presently, much of the electronic circuit manufacture is located off-shore to take advantage of cheap labor.
Briefly, these and other objectives and advantages are accomplished in one preferred embodiment of the present invention by an assembly which includes a conductive preform which has conductive leads with micron size connectors on each end so located as to connect with pads on chips and circuitry on a printed circuit board when a number of electronic circuit chips are placed on an upper surface on the conductive preform and the chips are urged toward the substrate or printed circuit board by a metallic cover containing an elastomeric pressure means with micron size connectors on a photo-resist film of the conductive preform causing a plurality of conductors of the conductive preform to connect with pads on the electronic circuit chips and traces on a substrate or printed circuit board.
In one preferred embodiment, the elastomeric pressure member is fabricated from a resilient elastomer material such as silastic No. 184 and includes a concentric, raised rib which encircles the periphery and projects toward the substrate or printed circuit board. With the elastomeric pressure means located within the cover, the flat surface of the elastomeric pressure means makes intimate contact with the interior surface of the cover when the cover is secured to the substrate or printed circuit board. When so assembled, the projection of the elastomeric means above the chips apply a force to the back of the chips which in turn applies pressure to pads of each chip urging the chip pads into intimate contact with the chip connector on one end of the conductive leads and at the same time longer projections on the elastomeric pressure means urge connectors on the other end of the conductive leads into intimate contact with circuitry on the substrate or printed circuit board traces. The chips have extremely flat surface on which the pads are formed and when pressed against connectors on the conductive leads supported on photo-resist film of the conductive preform with sufficient pressure some of the silver connectors on soft aluminum sink into the aluminum until all connectors of a particular chip make even contact with flat surface pads on the chip with the aluminum conforming to the more irregular surface of the substrate or printed circuit board below. The height of the interior of the cover is more or less 30% less than the thickness of the elastomeric pressure means plus projections over the chips and conductive leads on the preform in order that securing the cover to the substrate or printed circuit board produces and maintains sufficient pressure to maintain connection of chips to substrate in one embodiment and chip to chip and then to substrate in a second embodiment. The pressure on the backside of each chip being on the order of 1 to 2 pounds becomes amplified to a force of approximately 10,000 pounds per square inch distributed to each connector contacting the chip pads and each will be maintained in contact with pads of the chips by the elastomeric pressure means for a 20 year life as determined from accelerated life tests of the silastic used to form the elastomeric pressure means.
Similarly compressed, the raised ribs seals between the surfaces of the cover and the substrate or printed circuit board to hermetically enclose the electronic circuit chips within the assembly.
In those instances in which proper operation of an electronic circuit chip requires forming a connection to the chip's substrate, an alternative embodiment of the present invention further employs a substrate connection means which is applied to the surface of the elastomeric pressure means before contacting the chip. Such substrate connection means extends outward beyond the periphery of the chips with a connector to contact circuitry on the substrate or one of the printed circuit board's traces. In this alternative embodiment of the invention, a projection projects from the elastomeric pressure means to apply a force urging the substrate connection means into intimate contact with the circuitry on the substrate or a trace on a printed circuit board.
In addition to establishing a barrier to electrical conductivity, the elastomeric pressure means affects the flow of heat out of the electronic circuit chips into the atmosphere surrounding the assembly. Tests conducted indicated that heat transfer through the compressed elastomeric pressure means placed between the cover and operating chips is 15 degrees Celsius per Watt and compares favorably with that of chips in physical contact against a metallic cover, probably owing to the fact that chips do not make perfect contact with the cover whereas the compressed elastomeric pressure means makes intimate contact with the metallic cover so that a greater contact area off sets lower conductivity through the elastomer.
We have generally described a unique method for pressure connecting the input/output pads on an electronic circuit chip with electronic circuitry on a substrate or with other chips in one operation in such a fashion that continued pressure from an elastomeric pressure means maintains the pressure connections under shock or adverse conditions with the same operation serving to hermetically seal the assembly.